1. Field of the Invention
The invention relates to a method for communicating in a radio communication system. The invention furthermore relates to a radio station for communicating with other radio stations in a radio communication system.
2. Description of the Related Art
In radio communication systems information (for example speech, image information, video information, SMS (Short Message Service), MMS (Multimedia Messaging Service) or other data) is transmitted with the aid of electromagnetic waves between sending and receiving radio station via a radio interface. Depending on the actual embodiment of the radio communication system, the radio stations may in this case include different types of subscriber stations, radio access points or base stations. In such systems the electromagnetic waves are radiated using carrier frequencies that lie within the frequency band provided for the respective system.
In radio communication systems access by radio stations to the common transmission medium is controlled by multiple access/multiplex methods (MA: Multiple Access). With these multiple access methods the radio resources can be shared between the radio stations in the time domain (TDMA: Time Division Multiple Access), in the frequency domain (FDMA: Frequency Division Multiple Access), in the code domain (CDMA: Code Division Multiple Access) or in the space domain (SDMA: Space Division Multiple Access). Combinations of a number of these methods can also be used. Often (for example in GSM (Global System for Mobile Communication), TETRA (Terrestrial Trunked Radio), DECT (Digital European Cordless Telephone), UMTS (Universal Telecommunications System)) the transmission medium is subdivided into frequency and/or time channels. The radio resource is then a timeslot-frequency pair.
Radio communication systems are often embodied as cellular systems, e.g. in accordance with the GSM (Global System for Mobile Communication) or UMTS (Universal Mobile Telecommunications system) standard, having a network infrastructure, for example. of base stations, devices for monitoring and control of the base stations and further network-side devices. Frequencies at 900, 1800 and 1900 MHz are used for the cellular GSM mobile radio system. As well as these cellular, hierarchical radio networks organized on a wide area (supralocal) scale, there are also wireless local networks (WLANs: Wireless Local Area Networks) having, as a rule, a geographically much more limited radio coverage area. With a diameter of up to several hundred meters, the cells covered by the WLAN radio access points (AP: Access Point) are small compared to typical mobile radio cells. Examples of different standards for WLANs are HiperLAN, DECT, IEEE 802.11, Bluetooth and WATM.
In an ad hoc mode of a radio communication system, radio stations transmit messages to one another without the need for these messages to be forwarded by a central piece of equipment such as, say, a radio access point or a base station. In this case the connection between two radio stations is established either directly or, if greater distances are involved, via further radio stations which form relay stations for this connection. The radio stations of a self-organizing network of this type may be mobile radio stations (for example mobile stations of persons or in traffic vehicles) and/or predominantly stationary radio stations (for example computers, printers, household appliances).
Advantages of an ad hoc mode are that communication between radio stations is also possible in areas in which no radio coverage is provided by a network device. Furthermore, as a result of messages being forwarded by radio stations the transmitter power used by the radio stations can be reduced. Finally, with direct communication between equal-ranking radio stations (peer to peer), delays due to the sending of information via a base station are reduced. What is more, since the base station is not involved in the message transmission, network resources are saved.
Whereas with centrally controlled radio access methods radio resources are assigned to radio stations by a hierarchically superior unit such as, for example, a base station, in an ad hoc mode of a radio communication system the radio resources are accessed in a decentralized manner or, to put it another way, radio resources are assigned to radio stations on a decentralized basis. Access to the available radio resources is therefore coordinated on a self-organizing basis in an ad hoc mode. If there is no central equipment which informs the radio stations about the radio resources assigned to them, there is the problem that the radio stations have no knowledge of which radio resources are currently unassigned or, as the case may be, are not allocated to any radio station and so are available for communication purposes. This can lead to a situation in which a plurality of radio stations are using the same radio resources, thereby resulting in packet collisions. Furthermore, due to the lack of information of the radio stations about unassigned radio resources it is possible that a radio station, when sending a message to another radio station, uses a radio resource which the other radio station currently cannot use for receiving the message.